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Universal Soil Loss Equation (USLE)

Biotechnical and soil bioengineering slope stabilizationA practical guide for erosion control

by D. H. Gray and R. B. Sotir

Prepared by Walter Chen, Dept. of Civil Engineering, NTUTFor classroom teaching purpose

Prepared by Walter Chen ( 陳偉堯 )

Soil loss predictions 土壤流失預估

Semiempirical equation – universal soil loss equation (USLE) 半經驗公式

Developed by the USDA Agricultural Research Service in the early 1960s 美國農業部發展

Revised in the late 1970s as Agricultural Handbook 537

Prepared by Walter Chen ( 陳偉堯 )

USLE

Originally for predicting erosion losses from cropland east of the Rocky Mountains 洛磯山脈以東

Modified and adapted to different regions of the United States 經修改用於美國各地

Also for use at urban or highway construction sites

Prepared by Walter Chen ( 陳偉堯 )

A = R * K * LS * C * P

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Factors 因子

The climate factor (R) and the erodibility factor (K) only vary within one order of magnitude 降雨因子或氣候因子 (R) 及可侵蝕性因子 (K) 的變化範圍

The cover factor (C) and the topographic factor (LS) can vary over several orders of magnitude 土壤覆蓋因子 (C) 和地形因子 (LS) 的變化範圍

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

LS factor 地形因子

The topographic factor LS can be modified most easily by reducing slope length 降低邊坡長度 Benching 臺階化 Terracing 階地化 Live fascines 活捆樹枝 Contour wattles 等高線編枝

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

The C factor

Completely bare or fallow ground, C = 1 完全裸露地表

Even without canopy, C = 0.003 when ground cover = 95%

None of the other variables or factors are amenable to management with such dramatic results as the C factor

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Erosion control practice factor

Grade stabilization structures Level spreaders Diversions Interceptor berms Sediment basins

watch the discovery channel video segment on sediment basins

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

P factor

A slope left in a compacted and smooth condition is the most vulnerable to erosion – least amenable to vegetative establishment by seeding – why?

Trackwalking up and down slope is superior to trackwalking on contour – why?

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Limitations of USLE 限制

It is empirical (for sheet and rill erosion) – values of the factors?!

It predicts average annual soil loss based on 2-year, 6-hour rainfall Unusual storms could produce more sediments

It does not predict gully erosion It does not predict sediment delivery

soil loss from the upper portions of a slope may collect on the lower portions

Prepared by Walter Chen ( 陳偉堯 )

Erosion control principles 1/3

Fit the development plan to the site. Avoid extensive grading and earthwork in erosion prone areas.

Install hydraulic conveyance facilities to handle increased runoff.

Keep runoff velocities low. Divert runoff away from steep slopes and denuded

areas by constructing interceptor drains and berms.

Prepared by Walter Chen ( 陳偉堯 )

Erosion control principles 2/3

Save native site vegetation whenever possible. If vegetation must be removed, clear the site in

small workable increments. Limit the duration of exposure.

Protect cleared areas with mulches and temporary, fast growing herbaceous covers.

Prepared by Walter Chen ( 陳偉堯 )

Erosion control principles 3/3

Constructing sediments basins to prevent eroded soil or sediment from leaving the site.

Install erosion control measures as early as possible. Inspect and maintain control measures.